1. Field of the Invention
The present invention pertains generally to twist drills made of high speed steel (HSS), sintered metal HSS, cemented carbide or cermet, and in particular to improvements to reduce cutting resistance exerted thereon during drilling operation.
2. Prior Art
Twist drills made of HSS or sintered metal HSS have hitherto been developed as drills for effecting heavy-duty drilling operations. FIGS. 1 and 2 depict a conventional twist drill of such type which has a cylindrical body 1, a pair of spiral grooves or flutes 2 formed in the outer peripheral surface of the body 1 and a pair of lands each disposed between the pair of flutes 2. That wall portion of each flute 2 facing in the direction of rotation of the body 1 terminates at the forward end in a cutting edge or lip 3. Each spiral flute 2 is so formed that its wall is concavely shaped. In the drill for heavy-duty operations, the web thickness T of the drill body 1 is made greater than a HSS drill for normal drilling operations so that it is about 30% of the drill diameter, while the flute-width ratio at the forward end defined by the arc length A of the flute to the arc length B of the land is set to be about 0.7. Furthermore, at a cross-section of the drill, away from the forward end, the ratio of the arc length A.sub.1 of the flute to the arc length B.sub.1 of the land is set to about 1.16. With this construction, the torsional rigidity of the body 1 of the twist drill is considerably enhanced.
Furthermore, twist drills formed of cemented carbide or cermet have also been extensively employed for heavy-duty drilling operations. Such drills are superior in wear resistance to HSS drills, but, due to the inferior mechanical strength, e.g., transverse rupture strength, a greater web thickness and a smaller flute-width ratio are necessary. FIGS. 3 and 4 illustrate a prior art drill of such type as disclosed in Examined Japanese Patent Application Publication No. 61-30845, in which the symbols in common with those in FIGS. 1 and 2 denote the same or like parts. In this drill, the web thickness T and the flute-width ratio A/B are set to range between 20 and 35% of the drill diameter and between 0.4 and 0.8, respectively, while the flute-width ratio A.sub.1 /B.sub.1 at a cross-section away from the forward end is approximately 0.6.
In the twist drills of the afore-described types, however, there is always the problem that the drill body 1 is susceptible to breakage when subjected to heavy-duty drilling operations.
More specifically, chips or cuttings produced by the cutting lips 3 during a drilling operation are produced as if a sector-shaped folding fan were opened since their outer sides grow faster than the inner sides, and curl at their tip ends by the bottom 2a of the flute 2, i.e., by that portion of the flute wall where the distance between a line L perpendicular to a radial line N, connecting the axis O of the body 1 to a radially outermost end Q of the cutting lip 3, and the flute wall is greatest, so that the chips are broken at their roots by the resistance caused due to the curling. The chips thus formed are illustrated in FIG. 5, and are classified as "transition curled fractured type chips". In the above twist drills, the distance W between the line L and the bottom 2a of the flute 2 is made rather small in order to enhance torsional rigidity. As a result, the force exerted on the chips by the bottom 2a of the flute 2 acts in the direction opposite to the direction in which they grow, and hence thick chips, compressed strongly in the longitudinal direction, are produced. The addition of the relatively large force on the chips causes the drill body 1 to be subjected to a great cutting torque and thrust load.
Furthermore, in the above twist drills, the cross-sectional area of the flute 2 away from the forward end, which serves to discharge the chips, is rendered inevitably smaller, so that the chips may be jammed therein. This often causes the drill to break when the heavy-duty drilling operation is effected.